Amplifier circuitry having relatively low output impedance is necessary for driving large capacitive or resistive loads, such as the relatively low impedance of a telephone line, typically an impedance of about 600 ohms or less. In prior art, such amplifier circuitry typically is composed of a low impedance unity-gain buffer output stage fed by one or more voltage-gain stages. This amplifier circuitry typically is integrated in a semiconductor chip; whereas the load is situated off-chip.
For example, as shown in the textbook by P. R. Gray and R. G. Meyer, Analysis and Design of Analog Integrated Circuits (John Wiley & Sons, second edition, 1984) at page 759, Figure 12.43, a buffer stage for class B (push-pull) operation can be formed by a pair of error operational amplifiers (error op-amps) which are connected to receive feedback from a pair of serially connected output transistors in such a manner as to assure unity voltage gain. The operation is class AB (push-pull) in cases where in the quiescient (no signal) state both output transistors conduct current, but one of them does not conduct any current whenever the input voltage goes outside a narrow range, typically the range of about .+-.0.5 volt around ground potential. As known in the art, class AB operation in general is desirable from the standpoints of low standby power consumption and of no missing output signals for small input signals (no "dead zone") which would distort the output. Although such a buffer stage has the advantage of an output voltage swing (range) which is fully rail-to-rail (e.g. power supply V.sub. DD to power supply V.sub.SS) and the advantage of a low output impedance, it suffers from the disadvantage that any inequality in the voltage offsets of the operational amplifiers results in an undesirable non-conducting condition of one of the output transistors in the quiescent state, whereby the desired (push-pull) class AB operation is not feasible. Also, the circuit design is undesirably sensitive to semiconductor processing variations, so that either excessive power dissipation or circuit instability (oscillations) caused by poor feedback control over the quiescent current, or both, may result: Too high a quiescent current is wasteful of power, whereas too low a quiescent current can result in the above-mentioned circuit instability. That is, the circuit is not as reliable or robust as is desired.